In the prior art, Ag--Cu alloys containing Cu of 1 to 20% by weight, Ag--Ni alloys containing Ni of 1 to 20% by weight and the like are widely used as materials of electric contact such as a make and break contact incorporated into a relay or breaker, a slide contact incorporated into slide switches and a rotary slide contact mounted on motors.
However, the above materials are not recognized to have high arc resistance and abrasion resistance and sticking resistance becomes a problem. Particularly, the Ag--Cu alloy has a problem that its contact resistance increases and becomes unstable by the Cu oxide formed on its surface while it is used. Therefore, when a slide contact is formed of an Ag--Cu alloy and used as an outer peripheral contact piece of the commutator of a small-sized motor, the contact resistance varies, causing unstable rotation speed of the motor.
On the other hand, an Ag-metal oxide alloy is known as a contact material having a high sticking resistance.
For example, an Ag-manganese oxide alloy (refer to Japanese Patent Disclosure Nos. 51-136170 and 52-30217), Ag-indium oxide alloy (refer to Japanese Patent Disclosure No. 52-9625), Ag-zinc oxide alloy (refer to Japanese Patent Disclosure No. 54-149322) and Ag-oxide alloy in which the oxide indispensably contains an lithium oxide and additionally contains more than one of aluminium oxide, calcium oxide, magnesium oxide and silicon oxide (refer to Japanese Patent Disclosure No. 58-210133) are known.
The above Ag-metal oxide alloys are obtained by a method of heating an alloy of a certain composition of metal elements in an oxidizing atmosphere for a predetermined time to cause internal oxidation of the added elements other than the base material or Ag and precipitate a fine oxide of the added elements along the grain boundary of Ag.
The Ag-metal oxide alloy formed by the above internal oxidation method becomes a material whose sticking resistance or wear resistance is improved by the effect of the fine particles of the oxide of the added elements precipitated along the grain boundary of Ag when it is used as a contact material.
A slide contact material is widely used for various types of printers, cameras, VTRs in forms of a slide switch for a small current region or a rotary slide contact of a micromotor.
Various materials have been proposed for the above slide contact material. For example, an Ag--Cu alloy disclosed in Japanese Patent Disclosure No. 58-104139; an Ag--Sb alloy disclosed in Japanese Patent Disclosure No. 58-104141; an Ag--Zn alloy disclosed in Japanese Patent Disclosure No. 58-107441 and Ag--In alloy disclosed in Japanese Patent Disclosure No. 58-107458 are known.
Although not specified as a slide contact material, an Ag alloy containing Li and a rare earth element as indispensable components and a material obtained by subjecting the same to the internal oxidation process are known as a contact material which is good in resistance and wear resistance as is disclosed in Japanese Patent Publication No. 54-6008.
The above material has been developed mainly for a make and break contact material and is effectively used in the medium current region of approx. 1 to 100 A.
Recently, various types of electronic devices described above are required to be made further smaller and at the same time they are required to have a higher performance and higher reliability. Further, the devices are used in various environments, and for example, they may be sometimes used in an organic gas atmosphere containing a small amount of ammonia or formalin or in an atmosphere of high temperature and humidity.
In order to satisfy the above conditions, the electric contact incorporated into the above devices is required to have the following characteristics.
First, the electric contact incorporated in the device must be made smaller as the size of the device is reduced. At this time, the application current becomes small and the contact pressure tends to become smaller. For example, it is frequently used in a condition that current is set to 50 mA to 1 A and a pressure is set less than 10 g. Thus, when the application current becomes small and the contact pressure becomes small, abnormal contact resistance tends to occur in the contact portion, so that it becomes necessary to set the contact resistance of the a low contact material in contact portion in order to solve the above problem.
Further, when the contact pressure becomes small, a small arc occurs on the contact surface during the sliding operation in a slide contact, increasing wear of the material and therefore the material is required to have a higher arc resistance.
Further, as the contact is made smaller, the cross sectional area of the conductor portion becomes smaller. As a result, the total resistance of the contact becomes larger and the amount of heat generated in the contact while it is used increases, and the contact material is required to have a small resistivity.
The long service life of the contact is a factor of ensuring high reliability. Therefore, the contact material is required to be hard enough to be wear resistance.
Further, in the case of the rotary slide contact incorporated into a micromotor, for example, it is necessary to keep the contact resistance with time at a low and stable level in order to suppress the fluctuation of revolutions during the operation. In particular, the contact resistance thereof must be kept constant with time even when it is used for a long time in ammonia or an organic gas atmosphere, or in a high temperature and high humidity atmosphere. Therefore, the contact material is strongly required to have various corrosion resistances including oxidation resistance, sulfidization resistance, ammonia resistance and organic gas resistance.
In addition, recent micromotors tend to be operated at high speeds, for example, at a rotation speed of 5000 to 20000 rpm. However, in order to achieve the high speed operation with high stability, the slide contact incorporated in the motor must be formed by a material having a small friction coefficient and good lubricity.